The subject matter relates to a semiconductor design, and more particularly, to an output driver capable of ensuring the integrity of output data by controlling a slew rate of the output data.
Generally, a semiconductor memory device including double data rate synchronous DRAM (DDR SDRAM) outputs data to the outside using an internal clock signal synchronized with an external clock signal. In the semiconductor memory device, the data is synchronized with a rising edge and a falling edge of the external clock signal. Accordingly, an output driver is required in the semiconductor memory device in order to receive the internal clock signal and output data corresponding to the external clock signal.
FIG. 1 is a circuit diagram illustrating a conventional output driver.
Referring to FIG. 1, the output driver includes a pull-up pre-driving unit 110, a pull-up metal option unit 120, a pull-up main driving unit 130, a pull-down pre-driving unit 140, a pull-down metal option unit 150, and the pull-down main driving unit 160.
The pull-up pre-driving unit 110 generates the pull-up driving control signal CTR_PU in response to a pull-up data signal DAT_PU, and includes a PMOS transistor and an NMOS transistor which are series-connected between an external voltage terminal VDD and a ground voltage terminal VSS, and each receive the pull-up data signal DAT_PU through their respective gates.
The pull-up metal option unit 120 generates a desired signal by changing the pull-up driving control signal CTR_PU having a possibility of skew in accordance with process, voltage and temperature (PVT), and includes a first resistance R1 and a first switch SW1 parallel-connected between the pull-up pre-driving unit 110 and the pull-up main driving unit 130. Therefore, the skew, which may occur in the pull-up driving control signal CTR_PU, is changed using a metal option mask.
The pull-up main driving unit 130 performs a pull-up operation on the output terminal DOUT in response to the output signal of the pull-up metal option unit 120, and includes a PMOS transistor having a source and a drain connected between the external voltage terminal VDD and the output terminal DOUT, and a gate receiving an output signal of the pull-up metal option unit 120.
The pull-down pre-driving unit 140 generates a pull-down driving control signal CTR_PD in response to a pull-down data signal DAT_PD, and includes a PMOS transistor and an NMOS transistor which are series-connected between an external voltage terminal VDD and a ground voltage terminal VSS, and have gates receiving the pull-down data signal DAT_PD.
Similar to the pull-up metal option unit 120, the pull-down metal option unit 150 generates a desired signal by changing skew of the pull-down driving control signal CTR_PD using a metal option mask, and includes a second resistance R2 and a second switch SW2 parallel-connected between the pull-down pre-driving unit 140 and the pull-down main driving unit 160. Therefore, the skew, which may occur in the pull-down driving control signal CTR_PD, is changed using a metal option mask.
The pull-down main driving unit 160 performs a pull-down operation on the output terminal DOUT in response to the output signal of the pull-down metal option unit 150, and includes an NMOS transistor having a source and a drain connected between the ground voltage terminal VSS and the output terminal DOUT, and a gate receiving an output signal of the pull-down metal option unit 150.
Meanwhile, a third resistance R3 between the pull-up main driving unit 130 and the output terminal DOUT, and a fourth resistance R4 between the output terminal DOUT and the pull-down main driving unit 160 may be provided in order to output data from the output terminal DOUT more stably.
An operation of the output driver will be described briefly.
First, when the pull-up data signal DAT_PU becomes logic the ‘high’, the pull-up driving control signal CTR_PU becomes the logic ‘low’. Skew of the pull-up driving control signal CTR_PU is changed in accordance with the metal option mask of the pull-up metal option unit 120. The output signal of the pull-up metal option unit 120 turns on the PMOS transistor of the pull-up main driving unit 130. The pull-up operation is performed on the output terminal DOUT, and a logic ‘high’ signal is output through the output terminal DOUT.
When the pull-down data signal DAT_PD becomes the logic ‘low’, the pull-down driving control signal CTR_PD becomes the logic ‘high’. Skew of the pull-down driving control signal CTR_PD is changed in accordance with the metal option mask of the pull-down metal option unit 150, and the output signal of the pull-down metal option unit 150 turns on the NMOS transistor of the pull-down main driving unit 160. The pull-down operation is performed on the output terminal DOUT, and a logic ‘low’ signal is output through the output terminal DOUT.
As described above, the conventional output driver changes the skew of the pull-up driving control signal CTR_PU and the pull-down driving control signal CTR_PD using metal option masks. Therefore, more switches are provided to change the pull-up driving control signal CTR_PU and the pull-down driving control signal CTR_PD in accordance with variable skew. However, because a switch used in a metal option mask occupies a relatively large area, there is a limitation in increasing the number of the switches. Accordingly, the number of switches variable in accordance with skew should be limited, which means that the pull-up driving control signal CTR_PU and the pull-down driving control signal CTR_PD can not be variably secured by the conventional output driver.
Also, the pull-up metal option unit 120 and the pull-down metal option unit 150 have a constant resistance. Therefore, when, against expectation, skew occurs in the pull-up driving control signal CTR_PU and the pull-down driving control signal CTR_PD after the metal option mask, the desired pull-up and pull-down driving control signals CTR_PU and CTR_PD may not be secured.
The conventional output driver controls the slew rate of the pull-up driving control signal CTR_PU and the pull-down driving control signal CTR_PD by controlling the pull-up metal option unit 120 and the pull-down metal option unit 150. Thus, the pull-up main driving unit 130 and the pull-down main driving unit 160 are driven to control the slew rate of the output terminal DOUT. That is, in the conventional output driver, the full-up slew rate by which data is transited from the logic ‘low’ to the logic ‘high’ by the pull-up driving control signal CTR_PU is determined, and the full-down slew rate by which data is transited from the logic ‘high’ to the logic ‘low’ by the pull-down driving control signal CTR_PU is determined. That is, in the conventional output driver, the slew rates of the pull-up driving control signal CTR_PU and the pull-down driving control signal CTR_PD are in connection with the slew rate of the output terminal DOUT. However, the output driver with such a structure offers a narrow range of control for the slew rate of the pull-up driving control signal CTR_PU and the pull-down driving control signal CTR_PD.
As the operating frequency of the semiconductor memory device increases, the pull-up driving control signal CTR_PU and the pull-down driving control signal CTR_PD may not make a full swing in such a structure as in the conventional output driver. In such circumstances, the output driver is weakened especially against power noises and changes in accordance with PVT.
As described above, when the pull-up driving control signal CTR_PU and the pull-down driving control signal CTR_PD do not achieve a target slew rate, data output from the output terminal OUT also may accordingly have an undesirable slew rate.
That is, the data should be output in synchronization with the rising edge and the falling edge of the external clock signal, while the external clock signal is applied to the semiconductor memory device with an approximate duty rate of 50:50. Ideally, data should be output in accordance with the duty rate of the external clock signal. However, if the pull-up driving control signal CTR_PU and the pull-down driving control signal CTR_PD are not secured, the pull-up main driving unit 130 and the pull-down main driving unit 160 which operate in connection therewith may not output desired data. That is, data may not be output in accordance with the duty rate of the external clock signal. This lowers the reliability and the accuracy of the semiconductor memory device.